A light guide as herein contemplated may be either a single optical fiber, a group of optical fibers arrayed in a flat bundle or ribbon, or a light-conducting foil. In each instance the light guide has internally reflecting boundaries with a critical angle of reflection determined by the difference between the refractive indices of the guide substance and the surrounding medium. As is well known, light rays striking the guide boundary at a glancing angle, not exceeding the critical value, are totally reflected and thus do not leave the confines of the guide. In traveling along their transmission path, they bounce back and forth between opposite guide surfaces and eventually leave the exit end of the guide at an inclination to its axis which depends upon the angle of incidence.
Theoretically, at least, a ray may pass along the axis of a straight guide without internal reflection. Such a ray has the shortest transit time through the guide in comparison with rays undergoing reflection, the longest time being that of a ray exiting from the guide surface at the critical or guidance angle. These relative delays of light rays originating at a common modulated source result at the receiving end in a broadening of the pulses and thus in a distortion of the signal. That distortion, of course, increases with the length of the transmission path.
A variety of equalizers have already been suggested for dealing with this problem. One such equalizer, described by D. C. Gloge in an article entitled "Fiber-Delay Equalization by Carrier Drift in the Detector," Optoelectronics, vol. 5, 1973, pages 345-350, operates electronically on the electric pulses derived from the luminous signal at the receiving end; the light rays emerging at different angles from the exit end of an optical fiber are electronically detected in separate zones working into delay lines which introduce compensatory differences in transit time. Such a system, requiring active electronic components, is relatively complex and limited to specific radiation receivers.
In a commonly owned application filed by me jointly with Riccardo Vannucci on or about Feb. 18, 1977, Ser. No. 770,232, an optical signal-transmission system has been disclosed and claimed in which the equalization of the light paths is carried out with the aid of mirrors interposed between cascaded light guides angularly adjoining one another.
Other solutions, such as those suggested in U.S. Pat. Nos. 3,759,590 and 3,832,030, provide optical equalizers with refractive cones or lenses serving for a compensatory refraction of light rays incident at different angles.
The presence of three or more refractive bodies between confronting guide ends in systems of the last-mentioned type results in a significant attenuation of the luminous radiation, especially for slanting light rays which strike the surfaces of these bodies at almost a glancing angle and are therefore subject to heavy Fresnel losses.